1. Field of the Invention
This invention relates to the use of multiple beams and nozzles in direct material deposition (DMD) processes in order to increase the deposition rate without compromising material properties or dimensional accuracy.
2. Description of Related Art
Direct material deposition processes allow complex components to be efficiently fabricated in small lot sizes to meet the stringent requirements of the rapidly changing manufacturing environment. This process produces three-dimensional parts directly from a computer aided design (CAD) solid model. U.S. Pat. No. 4,323,756 teaches that complex, net-shaped objects can be built by sequential layer deposition of feedstock material in powder or wire form, whereby the material is directed into a focused laser beam, melted, and deposited onto a deposition substrate to generate solid objects of varying three-dimensional complexity in a layer-wise manner. Other prior art using this method includes "Using the Laser Engineered Net Shaping (LENS.TM.) Process to Produce Complex Components from a CAD Solid Model" by D. M. Keicher et al. in SPIE Conference, San Jose, Calif., January 1997. This method of direct material fabrication uses a single nozzle or powder delivery system that introduces a converging stream of powdered material into the laser beam at or near the beam's minimum diameter (i.e. focus or focal plane). The stream is at an angle off-normal to a deposition surface whereby uniform geometries of three-dimensional objects can be produced by providing computer controlled motion of the deposition surface relative to the laser beam. Experience has shown, however, that this nozzle design does not provide uniform flow independent of the translation direction. In addition, to achieve goal material properties, the deposition rate is sufficiently low, such that fabrication times required for even intermediate volume objects (100 cubic inches) are prohibitive.
U.S. Pat. No. 5,043,548 discloses a laser plasma spraying nozzle and method that permits high deposition rates and efficiencies of finely divided particles of a wide range of feed materials. This system uses powdered materials that are carried to the interaction regions via a carrier gas and lasers to melt these particles. However, this system relies solely on the use of a plasma to melt the particles before they are ever introduced to the deposition region. In fact, the carrier gas is often a mixture which promotes ionization, and, as such, the formation of a plasma. The plasma serves to melt the powder particles before they ever come into contact with the deposition substrate. In addition, the beam is diverging such that when it does impact the deposition substrate, the beam irradiance is sufficiently low so that no melting of the deposition substrate occurs. A great distance between the focal point of the laser and the central portion of the plasma is maintained to prevent the substrate from melting. This distance, ranging from 1-6 inches, is a characteristic of this apparatus. The materials are deposited in either a liquid or gaseous state. This design provides a unique method for coating parts; however, it has never been intended for fabrication of multilayered parts. Due to the diverging nature of the powder material, this plasma technique fails to provide the, feature definition necessary for fabricating complex, net-shaped objects.
Another nozzle design is shown in U.S. Pat. No. 4,724,299. This nozzle design requires the powder to be delivered from an annular source that is coaxial with a single laser beam. This design provides a uniform feed of powder to the cladding region, a laser used as an energy source to melt the powder that is to be deposited, and a powder distribution system. However, this system requires that the powder distribution system be contained within the nozzle assembly.
The nozzle design of the '299 patent is very specific to the laser cladding application. The term laser cladding is used specifically to imply surface modification and not the direct fabrication method. More importantly, the design relies on having an annular powder distribution channel to deliver the powder to the deposition region. The annular powder distribution region provides powder to the focused laser beam from all directions and does not concentrate the powder for a tightly focused powder stream. For a single laser beam that is coaxial to the powder flow, this nozzle should work well to provide a uniform layer, however, there is concern that the powder distribution at the deposition surface is greatest at the center of the deposition region, causing it to diminish radially away from the center of the deposition spot. With this change in powder volume uniformity, the inclusion of multiple beams will certainly result in varying line size for parallel deposited lines.
U.S. Pat. No. 4,323,756 also covers the direct metal deposition (DMD) process. This technique uses both wires and powders as filler material. It also uses a single laser beam to deposit the various materials. This patent teaches that the volume of the feedstock material must be less than that of the melted substrate material. However, this reduces the rate of deposition and increases the time to produce parts. The requirement to limit the volume of the feedstock material to be less than that of the melted substrate material was driven by the desire to remove impurities and obtain epitaxial growth. Instead of removing impurities by continuously remelting the previously deposited materials, impurities can be efficiently eliminated by performing the deposition in a controlled atmosphere environment, such as a glove box. Furthermore, expitaxial growth is not desired in most three-dimensional parts, since it may result in anisotropic material characteristics. For most general applications, uniform material properties are desired that do not limit the feedstock volume to be less than that of the deposition substrate melted region.
The above single laser nozzle's side design lacks the ability to increase the deposition rate of powder without sacrificing vital process conditions, including reduced residual stress, enhanced material properties, process time, and good dimensional repeatability, as well as feature definition. Also, this nozzle design does not provide uniform flow independent of the translation direction. Therefore, such nozzle designs are not suitable for mass 3-D net-shape production.
U.S. Pat. No. 5,578,227 contains similarities to the present invention, such as the use of a positioning system to direct the location of deposition, and the use of a laser to deposit the feedstock material. However, the '227 patent only uses a single laser beam for the deposition process, which uses wire as the feedstock material. This patent also claims that the laser causes the feedstock material to bond to the previously deposited layer without substantially altering the cross-section of the newly deposited material. Such a continuous form of material would appear to be prone to substantial problems of warpage and distortion of the deposited layers due to an incomplete melting of the feedstock material. For the powder deposition processes, the feedstock material is completely consumed within the 3-D net shape, with the powder's cross-section being substantially altered.
Accordingly, there exists a need for improved material deposition nozzles for the laser assisted deposition process.